Electrical strain insulator and method of making same



July 19, 1966 M. JAcQUiER 3,261,910

ELECTRICAL STRAIN INSULATOR AND METHOD OF MAKING SAME Filed Aug. 12, 1964 United States Patent O tion Filed Aug. 12, 1964, Ser. No. 389,104 Claims priority, application France, Aug. 20, 1963, 945,145; Jan. 16,1964, 960,548 7 Claims. (Cl. 174-178) The present invention relates .to electrical strain insulators.

Strain insulators are already known which comprise two metallic end pieces provided with anchoring means. The end pieces are linked together by a connecting piece formed of filiform elements such as glass fibers bound together by a resin. However, none of these known strain insulators presents the advantages corresponding to the high characteristics of their components. This is due to lthe fact that glass fibers present a low lstretch index and a low resistance to shearing stresses, and that in the prior strain insulators the problem of securing the metallic end pieces to the connecting piece has not been solved satisfactorily. It is the purpose of this invention to obvlate such disadvantages. p

One embodiment utilizing the principles of this invention provides a method of making strain insulators comprising -two metallic end pieces each provided with a shoulder forming portion and a connecting piece; joining these end pieces such that iiliform elements assembled 1n strands are wound and stretched around said shoulders and the'connecting piece So as to forma wrapping or sleeve comprising a cylindrical portion terminating at both ends in a cap, the shape of which approachesthat of a portion of a sphere. The particular shape of the shoulders is the most important feature which renders possible the making of strain insulators, presenting a very high linear strength. The most advantageous shape of the shoulders is a composite shape of a portion of a sphere and of a portion of ellipsoid, the revolution axis of which is the longitudinal axis of the strain member.

Accordingly, it is an object of y-this invention to provide an electrical strain insulator having a high linear strength and high insulating properties but of relatively small cross-section and weight.

It is another object of this invention to provide a novel method for making an electrical strain insulator.

Other objects and advantages will become apparent from a study of the following specification and drawings in which:

FIG. 1 is a longitudinal sectional View of a support element supporting the winding of the strain insulator of the invention.

FIG. 2 is a longitudinal sectional view of the strain insulator ofthe invention.

Referring to FIGS. l and 2, reference numerals 2 and 3 designate two metallic end pieces partly engaged with a tube or connecting piece 1 and forming with the latter a cylindrical rod having two shoulders 4 and 5. The end pieces 2`and 3 also include anchoring portions 6 and 7. Tube 1 is filled with an insulating material 12 which can be in a gaseous, liquid or solid state. The shoulders 4 and 5 have preferably a shape which is intermediate between a spherical and ellipsoidal shape, the revolution axis of which is the longitudinal axis of the strain insulator. The support-element formed by the two end pieces and `the connecting piece is provided with a wrapping or sleeve 8 made of a fiber glass roving continuously wound back and forth from end to end of said support-element.

According to the invention the winding process is executed as follows: The support-element is disposed on a holder located in front of a winding machine supplied 3,261,910 Patented July 19', 1966 ICC with fibrous strands. The end of a strand is secured, by any known means, yto the support-element. Then, the support-element is rotated while the strandis regularly, rapidly and continuously displaced from one extreme end to lthe other end of the support-element. The roving is wound in very long pitched spirals, thus forming a cylindrical sheet of wound strands ending in two caps of superimposed strands upon -the shoulders, each strand following a geodesic curve on said shoulders.

Such an arrangement presents numerous advantages. The strands have no tendency to be displaced on the shoulders 4 and 5 since there is no lateral gliding force component, owing to the fact that the strands follow geodesic curves on the shoulders. Further, the shearing stresses are reduced to a minimum value and use is made of the most interesting property of the glass fibers, i.e. their tensile strength. Such an arrangement allows a reduced thickness of the fiber layer for a given strain resistance per mm?. In fact, it is important that all the strands of the sheet are submitted to substantially similar tensile stresses because of the low stretch coeflicient of such strands; since there is no equalizationof stresses by differential stretch, the layers, working under different tensile stresses, would break one after the other. This happens in strain insulators comprising a thick ysleeve constituted of an important number of superimposed sheets of fiber strands. In this case, the inner and outer fibers work under different tensile stresses and the thicker the sheet the greater the difference between the stresses applied to the inner and outer strands respectively.

In the strain insulator according to this invention, the thickness of the roving wound on the support is only of about one-tenth of lthe diameter of tube 1, which serves as a mandrel, and `this gives the greatest tensile strength to the strain insulator. It should be noted that to obtain the best result, the pitch of the winding should be as long as possible. This is obtained by adjusting the rotary speed of the support element in function of the diameter and length of said support element.

Advantageously, the filiform elements used in such a winding can be coated with lthermosetting resin in order to bind together -the turns of the winding after polymerization. This coating can be applied before, during, or even after the winding process. It is also advantageous to use untwisted fibrous strands having a diameter under l0 microns. Polyester or epoxy resin with their usual adjuvants may be used as thermoset-ting resins. The coating resin liber glass weight ratio is advantageously comprised between 10% and 30%, the recommended ratio being around 20%. In case the strain insulator is located in an insulating fluid medium, it is possible either to omit coating the fibrous strands or to provide such a coating only on a par-t of the winding, for example, on the end portions of the strain insulator.

In order to obtain the highest insulating efficiency, the tube 1 is made by winding glass fibers on a mandrel, said fibers being wound with the same pitch and bound with the same coating as sleeve or wrapping 8. Thus, a more close connection is obtained ybetween the sleeve 8 and the support-element 1 eliminating the risk of any striking of electric arc in the medium constituting the junction between Isaid sleeve and said support element.

The glass fiber of the winding constituting tube 1 has a section under 10 microns.

Two channels 9 and 10 pass through the metallic end pieces 4 and 5, and the inside of the tube 1 is filled through the channels 9 and 10 with an insulating medium 11 which adheres on the inner surface of the tube 1.

A strain insulator manufactured according to the method of this invention presents a tensile strength higher than kg./mm.2 for the wound part of the insulator, i.e. a strain insulator having an overall diameter of only S cm.

is capable of sustaining the stresses imposed by a 28 ton load. With regard to the insulating properties, such insulator can resist, without destructive breakdown, a high voltage such as the passing round arc voltage.

The strain insulator can include discharge guards regularly spaced along the cylindrical part of the member secured to the latter by sticking, or incorporated among the strands of the winding.

The strain insulator may be coated with resin which is not affected by the thermic effect of an electric arc, for example, a resin including polytetrauoroethylene. It is also possible to dispose the strain insulator within an insulating casing made of a ceramic. Suitable means are provided for ensuring the tightness of the sealing joints disposed between said casing and the ends of the strain member and the insulating material filling the space between the casing and the strain insulator. Further, the tube 1 may be replaced by a rod provided at both the ends with a recess wherein the end pieces are partly engaged.

Although several embodiments of the invention have been depicted and described, it will lbe apparent that these embodiments are illustrative in nature and that a number of modifications in the apparatus and variations in its end use may be effected without departing from the spirit or scope of the invention as defined in the appended claims.

I claim:

1. An electrical strain insulator comprising two metallic end pieces each provided with a shoulder forming portion of substantially hemispherical shape, a connecting piece joining said end pieces, said end pieces and connecting piece forming a support element, a fiber glass roving having a strand diameter substantially smaller than the diameter of said connecting piece wound around said support element and forming a wrapping of substantially uniform thickness, said fiber glass having a long pitch spiral and following the geodesic lines on said shoulders for securing together said end pieces and said connecting piece.

2. An electrical strain insulator comprising two substantially hemispherical end pieces terminating at their apices in an anchor forming shank portion, a cylindrical connecting piece having substantially the sarne diameter as said end pieces disposed therebetween for forming therewith a support element, -a fiber glass roving having a strand diameter substantially smaller than the diameter of said connecting piece wound around said support element with a longitudinal pitch and following the geodesic lines on said end pieces, said roving having a substantially uniform thickness for covering and securing together said end pieces and said connecting piece.

3. An electrical strain insulator as claimed in claim 2 wherein said end pieces have a shape intermediate that of a portion of a sphere and that of a portion of an ellipsoid.

4. An electrical strain insulator as claimed in claim 2 wherein said Wrapping has a thickness of about one-tenth of the total diameter of the cylindrical connecting piece.

5. An electrical strain insulator comprising two substantially hemispherical end pieces each having at their apieces an anchor forming shank portion, and at their opposite sides a -cylindrical stepped portion of reduced diameter, a cylindrical connecting piece having substantially the same diameter as said hemispherical end pieces and provided at both ends with an opening having substantially the same diameter as said cylindrical stepped portions, each end piece being engaged in one of said openings by its stepped portion, said end pieces and said connecting piece forming a support element, a fiber glass roving having a strand diameter substantially smaller than the diameter of said connecting piece wound around said support element with a longitudinal pitch and following the geodesic lines on said end pieces, said roving having a substantially uniform thickness for covering and securing together said end pieces and said connecting piece.

6. An electrical strain insulator as claimed in claim S, wherein said connecting piece is a tubular element made of ber glass roving windings Ibound by a resin, said winding having substantially the same pitch as the winding of said wrapping.

7. An electrical strain insulator as claimed in claim 1, wherein said end pieces are provided with a channel for filling the inside of said connecting piece.

References Cited by the Examiner UNITED STATES PATENTS 706,194 8/1902 McCarthy 174-179 735,611 8/1903 Steinberger 174-184 X 1,170,723 2/1916 Allerding 174-180 1,730,327 10/1929 Kempton 174-178 2,723,705 11/ 1955 Collins. 2,732,423 l/ 1956 Morrison. 2,924,643 2/1960 Barnes 174-178 FOREIGN PATENTS 1,040,850 5/ 1953 France.

138,539 2/ 1920 Great Britain.

775,773 5/ 1957 Great Britain.

821,101 9/1959 Great Britain.

915,052 1/1963 Great Britain.

LARAMIE E. ASKIN, Primary Examiner.

ROBERT K. SCHAEFER, Examiner. 

1. AN ELECTRICAL STRAIN INSULATOR COMPRISING TWO METALLIC END PIECES EACH PROVIDED WITH A SHOULDER FORMING PORTION OF SUBSTANTIALLY HEMISPHERICAL SHAPE, A CONNECTING PIECE JOINING SAID END PIECES, SAID END PIECES AND CONNECTING PIECE FORMING A SUPPORT ELEMENT, A FIBER GLASS ROVING HAVING A STRAND DIAMETER SUBSTANTIALLY SMALLER THAN THE DIAMETER OF SAID CONNECTING PIECE WOUND AROUND SAID SUPPORT ELEMENT AND FORMING A WRAPPING OF SUBSTANTIALLY UNIFORM THICKNESS, SAID FIBER GLASS HAVING A LONG PITCH SPIRAL AND FOLLOWING THE GEODESIC LINES ON SAID SHOULDERS FOR SECURING TOGETHER SAID END PIECES AND SAID CONNECTING PIECE. 